The Scoop Or Double Knee Bend - What, Why & How

The double knee bend might be more accurately called the naturally occurring temporary cessation of knee extension to facilitate the preservation of balance and repositioning of the body to optimize vertical propulsion… but that’s a bit of a mouthful.
The double knee bend occurs naturally if you position yourself correctly when entering the second pull, and you drive forcefully against the ground with your legs along with the hip extension. You do not need to intentionally scoop your knees under the bar, and doing so usually slows the bar and disrupts your balance.
The double knee bend happens for 2 primary reasons. First, the hamstrings cross both the hip and knee—when you contract the hamstrings forcefully for the second pull, they want to both extend the hip and flex the knee, which temporarily freezes or slightly reduces the knee angle.
Second, the body is repositioning to allow the application of vertical force with the legs—to do this, the hips need to move forward under the shoulders, which means the knees that are stuck in a partially bent position move forward under the bar as well.
Once this shift in position has occurred, the knees naturally continue their extension with the hips.
If you reduce or eliminate this vertical leg drive effort, you’ll execute horizontally-oriented hip extension like you’d see in a kettlebell swing—the knees will not scoop forward.
A simple demonstration is to perform a vertical jump from a simulated second pull position. Stand with the shins approximately vertical, the hips back, and the shoulders in front of the knees. Directly from this position, without a countermovement, jump straight up.
You’ll find the knees naturally and unavoidably shift forward, exactly as they do in a snatch or clean.